Pigment microparticles are used by dispersing them into a dispersing medium such as water and an organic solvent. However, in general, as the size of pigment microparticles becomes smaller, dispersion into a dispersing medium becomes more difficult; and as a result, an equipment requiring large energy such as a bead mill and a high pressure homogenizer is necessary for dispersion, or there is a problem of dispersion stability due to reaggregation of microparticles once dispersed. Therefore, to homogeneously disperse pigment microparticles, such a method in which surface of pigment microparticles is modified to increase affinity with a dispersing medium or with a dispersing agent contained in a dispersing medium has been known in the art (Patent Document 1 and Patent Document 2).
In addition, a method in which, after a water-swelled organic pigment paste is added into a reaction medium for sulfonation to disperse the pigment, water is removed and then a sulfonic acid group (sulfo group) is introduced onto particle surface of the organic pigment by a sulfonating agent (Patent Document 3), a method in which phthalocyanine pigment particles are dispersed into an aqueous solution of a water-soluble phthalocyanine derivative, which is obtained by introducing an acidic functional group or its salt into phthalocyanine, thereby adsorbing the water-soluble phthalocyanine derivative onto surface of the phthalocyanine pigment particles (Patent Document 4), and a method in which an aqueous sodium hydroxide is added dropwise into an aqueous slurry solution containing copper phthalocyanine to control at pH 12.0, and then the resulting mixture is irradiated with ultrasonic wave of 38 KHz at 150 W from ultrasonic transducer 2 of ultrasonic oscillator 1 under an ozone-containing oxygen gas stream at the flow rate of 0.5 litter per minute to introduce a hydrophilic functional group onto surface of copper phtalocyanine microparticles (Patent Document 5), have been known.
However, in surface treatment of aggregated particles or particles once dried, it is general that surface treatment must be done after redispersion of intended particles into a solvent. Because of this, large energy is necessary to disperse uniformly the particles whose surface is not treated; and in addition, it is difficult to treat surface of each particle uniformly. Further, in the case that surface modification is done after formation of microparticles by atomization, large energy is necessary in each step. Still further, to carry out intended treatment in each step, different reagents are necessary in respective steps thereby leading to increase in cost and wasting large amount of resources.
There is a method in which an organic pigment solution obtained by dissolving the pigment into a good solvent to the pigment is mixed with a solvent being capable of a poor solvent to the pigment thereby effecting separating of the pigment into a mixed solution as particles with surface of the particles being modified almost at the same time substantively; in this regard, disclosed is a method for producing surface-treated pigment, wherein a mixture, obtained by separating into a water from a strongly acidic solution of a poorly water-soluble compound having an acidic functional group bonded to its dye moiety and a compound that is a constituting component of an organic pigment, is adsorbed onto particle surface of the organic pigment (Patent Document 7). However, this method comprises a step of producing a compound having a modification group in its dye moiety and steps of dissolution and reseparating of a mixture of the pigment with the compound having a modification group in its dye moiety; and thus the method has problems of increased number of steps, increased cost, and so on. It is to be noted that the modification group means a functional group or a radical that is introduced or bonded at least onto particle surface.
In addition, there is a method in which an organic pigment solution obtained by dissolving the pigment into a first solvent that is a good solvent to the pigment is mixed with a second solvent being capable of a poor solvent to the pigment thereby effecting separating of the organic pigment as nanoparticles into a mixed solution, then a polymer having mass-average molecular weight of 1000 or higher is introduced and at the same time a surface-modification group is substantially introduced (Patent Document 8); in this method, however, the surface-modification group inevitably contains the polymer; and thus, this method has such a problem that both the surface-modification group and the polymer need to be selected in accordance with a dispersing medium and a dispersing agent of produced particles.
According to the invention by the present applicant (Patent Document 6), pigment microparticles are produced by using an equipment wherein a minute space is kept between two processing surfaces being capable of approaching to and separating from each other and rotating relative to each other, wherein this minute space kept between two processing surfaces is used as a flow path of a fluid to be processed thereby forming a thin film fluid (forced thin film) of the fluid to be processed, wherein pigment microparticles are formed in this thin film fluid (forced thin film); however, even by using such an equipment, easy dispersiveness of pigment microparticles could not be established yet.